Iris imaging using reflection from the eye

ABSTRACT

A rapid iris acquisition, tracking, and imaging system can be used at longer standoff distances and over larger capture volumes, without the active cooperation of subjects. The captured iris images can be used for biometric identification. Light illuminates the subjects&#39; eyes. Reflections from the eyes are used to steer a high resolution camera to the eyes in order to capture images of the irises.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application (a) is a continuation of U.S. patent application Ser.No. 11/765,401, filed Jun. 19, 2007, entitled “Iris Imaging UsingReflection From The Eye,” which claims priority under 35 U.S.C. §119(e)to U.S. Provisional Patent Application Ser. No. 60/815,000, filed Jun.19, 2006, entitled “Iris Imaging Using Reflection From The Eye,” and (b)is a continuation-in-part of U.S. patent application Ser. No.11/297,578, filed on Dec. 7, 2005, entitled “Iris Imaging UsingReflection From The Eye,” which claims priority under 35 U.S.C. §119(e)from both U.S. Provisional Patent Application Ser. No. 60/654,638,“Biometric Identification and Iris Imaging Using RetinalRetro-Reflection,” filed Feb. 17, 2005 and U.S. Provisional PatentApplication Ser. No. 60/634,331, “Adaptive Optics (AO) Imaging Appliedto Biometric Identification Using Iris Imaging,” filed Dec. 7, 2004. Thesubject matter of all of the foregoing is incorporated herein byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to imaging of the human iris, as may be used forbiometric identification.

2. Description of the Related Art

As traditional forms of personal identification become vulnerable toadvancing technology, biometric identification is increasingly seen as aviable approach to personal identification. Techniques such as voicerecognition, fingerprinting, and iris imaging rely on physical personaltraits that are difficult to change or duplicate.

However, biometric identification via iris imaging typically requires ahigh resolution image of the iris in order to resolve the fine detailsnecessary to make a positive identification. An image of an iris withapproximately 200 micron or better spatial resolution typically isrequired to uniquely distinguish the fine muscle structure of humanirises, as may be required for identification purposes. In systems wherethe subject is actively cooperating, conditions such as illuminationgeometry, camera resolution, exposure time, and wavelength of light canbe optimized in order to capture a high contrast image of the finestructure of the iris. Existing systems typically require a subject tohold his head in a specific position while staring at the iris imagingcamera from close proximity and at a nearly head-on aspect. Althoughrecent advances have been made in iris imaging, the task of capturingsufficiently high resolution images of the human iris generally stillrequires a fair degree of active cooperation from the subject.

For example, a system using commercial color CCD technology (e.g., 5megapixels) would typically have a field of view of approximately 15 cmat a 1 m standoff range, yielding a spatial resolution of approximately75 microns per pixel at the 1 m standoff range. Thus, the subject wouldhave to be within approximately 1 m of the camera and would have toposition his iris within the 15 cm field of view for a long enoughperiod of time in order for the camera to focus and capture an adequateresolution image of the iris. This typically requires the subject'sactive cooperation. The situation becomes significantly worse at longerstandoffs. For example, if the same camera were used at a standoff of 10m, maintaining the same angular resolution would result in a spatialresolution of 750 μm per pixel, which is unacceptable. On the otherhand, maintaining a spatial resolution of 75 μm per pixel would resultin a 15 cm wide field of view at 10 m. Keeping the iris within thisfield of view is also very difficult.

The “capture volume” of an iris imaging system is the volume over whichthe iris imaging system can capture iris images of sufficiently highresolution. The CCD-based system described above and other similartraditional systems have a small capture volume—so small as to maketraditional iris imaging systems unsuitable for use in uncooperativesituations, such as iris imaging over large groups of people, overlonger standoff distances, or for covert identification applications.For example, it may be desirable to capture iris images of subjects asthey walk through a portal, such as a metal detector, or in places likeairports, train stations, border crossings, secure building entrancesand the like. The high-resolution and longer standoff requirements inthese applications place significant challenges on iris imaging systemsthat cannot be met by current designs. The capture volume and standoffcapabilities of current iris imaging systems are not large enough toefficiently address these types of situations.

Therefore, there is a need for iris imaging systems that have largercapture volumes and/or can be used at longer standoff distances.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations of the prior art byproviding a rapid iris imaging system that can be used at longerstandoff distances and over larger capture volumes, without the activecooperation of subjects. Light illuminates the subjects' eyes.Reflection from the eyes (e.g., retro-reflection from the retina orglint reflection from the cornea) is used to steer (and preferably alsofocus) a high resolution camera to the eyes in order to capture imagesof the irises. Real-time steering and focus correction may extend theusable exposure time, thus allowing good images under lower illuminationlevels than otherwise possible. Other methods may also be envisaged forreal-time control of steering and focus.

In one embodiment, the iris imaging system includes an imagingsubsystem. The imaging subsystem includes a camera, a light source and afine tracking system. The camera captures images of irises withsufficient resolution for biometric identification. The light sourceproduces light that illuminates eyes within a capture volume. The finetracking system steers the camera to eyes, based on a reflection fromthe eyes, preferably either a retro-reflection or a glint reflection.

In one approach, the fine tracking system includes an adaptive opticsloop that is driven by the reflected light. For example, the adaptiveoptics loop can include a deformable mirror, a wavefront sensor and acontroller. The wavefront sensor senses the wavefront of the reflectedlight and a controller drives the deformable mirror based on the sensedwavefront. The deformable mirror corrects the incoming wavefront, thussteering the camera to the eye (i.e., correction of tip and tiltwavefront errors). The deformable mirror may also focus the camera(i.e., correction of focus-error). In this way, the imaging subsystemcan acquire iris images, even without the subject's active cooperation.

The iris imaging system may also include an acquisition subsystem thatidentifies the approximate location of subjects within a capture volume.For example, a wide field of view acquisition subsystem may be coupledwith a narrower field of view imaging subsystem. The acquisitionsubsystem identifies the approximate location of subjects, and theimaging subsystem slews from one subject to the next to acquire imagesof their irises. A controller coordinates the two subsystems. In oneapproach, the acquisition subsystem identifies the approximate locationof subjects based on retro-reflections from the subjects' eyes. This isconvenient since the circular shape of the eye pupil allows one toeasily distinguish retro-reflections from the eye from other lightsources. The two subsystems may be partially or fully integrated. Forexample, they may be optically aligned so that they are both looking inthe same general direction, although the acquisition subsystem typicallywill have a much larger field of view than the imaging subsystem.

According to one embodiment, illumination can be used for threeprinciple purposes in the iris imaging system. Light can be used by theacquisition subsystem to identify the approximate location of subjectswithin a capture volume. Light can be used by the fine trackingsubsystem to drive the adaptive optics loop to steer the camera to theeye. Light can also be used to illuminate the iris for imaging by theimaging subsystem. In some variations, the light used for one of thesepurposes is also used for another of these purposes. In some furthervariations, light for at least two of these purposes is separated bywavelength, angle (i.e., propagation direction), space, time,polarization, modulation, or some combination of these attributes.

Other aspects of the invention include methods corresponding to thedevices and systems described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention has other advantages and features which will be morereadily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is an illustration of an iris imaging system according to thepresent invention.

FIG. 2 is an illustration of another iris imaging system according tothe present invention, based on retro-reflection from the eye.

FIG. 3 is an illustration of retro-reflection from the eye.

FIG. 4 is an illustration of a typical reflectance spectrum of a humaneye.

FIG. 5 is an illustration of another iris imaging system according tothe present invention, based on glint from the eye.

FIG. 6 is an illustration of an iris imaging system where light isdifferentiated by spatial location.

FIG. 7 is an illustration of an iris imaging system where light isdifferentiated by wavelength.

FIG. 8 is an illustration of another iris imaging system where light isdifferentiated by wavelength.

FIG. 9 is an illustration of two examples of iris imaging systems wherelight is differentiated in time.

The figures depict embodiments of the present invention for purposes ofillustration only. One skilled in the art will readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles of the invention described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an illustration of an iris imaging system according to thepresent invention. The iris imaging system includes an imaging subsystem200 and, optionally, an acquisition subsystem 100. The system isdesigned to capture iris images of many eyes 134 over a large capturevolume 50, typically without the active cooperation of the subjects. Inone application, the subjects are passing through a portal 60 (such as adoorway or metal detector), the capture volume 50 is the entrancewayleading up to the portal, and the iris imaging system captures irisimages as the subjects pass through the capture volume. In manyapplications, the capture volume can be defined based on a portal orother bottleneck for the flow of people. Examples include corridors,turnstiles, toll booths, elevator doors, escalators and parking garageentrances. Other examples include checkout lines or other queues,crosswalks, sidewalks and roadways.

This situation typically is “uncooperative,” meaning that the subjectsare not actively cooperating in the iris imaging. For example, they arenot placing their heads into a device to allow capture of iris images.Rather, they are simply walking through the portal and the systemcaptures their iris images as they do so. They may even be unaware thatthe system is doing so. If stealth is important, the wavelengths shouldbe chosen to be non-visible.

The imaging subsystem 200 captures the iris images for each subject.However, in order to obtain sufficient resolution in the iris image, theimaging subsystem 200 has a fairly narrow field of view 230. Therefore,in order to cover the entire capture volume, the imaging subsystem 200is actively steered from one subject to the next. Coarse tracking ofsubjects can be achieved in many different ways. In FIG. 1, anacquisition subsystem 100 with a wide field of view 130 is used toidentify the approximate location of each subject. This information isused to coarsely steer the imaging subsystem 200 to the general vicinityof the subject. Once in the general vicinity, fine tracking is achievedby illuminating the subject's eye with an optical beam and steering theimaging subsystem 200 to the eye based on a reflection from thesubject's eye. Examples of eye reflections include retro-reflection fromthe retina and glint reflection from the corneal surface. The eyereflection can also be used to focus the imaging subsystem 200 on theiris to capture the high resolution image. The tracking (and focus)occurs fairly rapidly in real-time if a large capture volume andthroughput of subjects is to be accommodated.

Different devices can be used for the acquisition subsystem 100 and forthe imaging subsystem 200. The acquisition subsystem 100 can also bebased on tracking subjects using reflection from their eyes.Alternately, it can be based on completely different mechanisms. Forexample, the acquisition subsystem 100 might capture conventionaldigital images of the capture volume. Software is then used to identifywhich parts of each captured image represent humans and/or which part ofeach human is his face or eyes. Frame to frame comparisons can be usedto track movement of subjects. Stereoscopic systems (based on eyereflection, conventional imaging or other approaches) can be used totriangulate subject positions within the capture volume.

In FIG. 1, the acquisition subsystem 100 is shown as a single box with awide field of view 130. This is merely a representation. The acquisitionsubsystem 100 is not limited to a single box. In the stereoscopicexample, equipment is positioned at different locations in order tocapture different viewpoints. Even if a stereoscopic approach is notused, multiple cameras can still be used advantageously, for example tomore efficiently cover the entire capture volume 50.

The wide field of view 130 also need not be implemented literally asshown in FIG. 1. Each acquisition camera(s) may have a wide field ofview that covers the entire capture volume 50, as shown in FIG. 1.Alternately, each acquisition camera may cover less than the entirecapture volume 50, but the cameras together cover the entire capturevolume 50. In addition, the cameras may be scanning rather than staringand their instantaneous fields of view may be smaller than the capturevolume 50. At any instant in time, only a fraction of the entire capturevolume is covered but, over time, the entire capture volume is covered.

As a final example, the acquisition subsystem 100 may not be based oncameras at all. Other types of position sensors or intrusion sensors maybe used to determine the location of subjects. For example, the capturevolume 50 may be covered by a grid of light beams. The position ofsubjects is determined by the subjects' breaking the light beams. In adifferent approach, floor mounted pressure pads may be used to determinesubject positions. Sonar, radar, lidar, and thermal detection or imagingare examples of other technologies that can be used to determine subjectpositions. For certain types of sensors, the term “field of view” maynot even be applicable, so long as the acquisition subsystem 100 issufficient to cover the capture volume 50.

Controller 190 coordinates the two subsystems. The information from theacquisition subsystem 100 is used by the imaging subsystem 200 (viacontroller 190) to coarsely steer the narrow field of view 230 fromsubject to subject. As with the acquisition subsystem 100, manydifferent designs for the imaging subsystem 200 are also possible. Inone approach, conventional devices such as steering mirrors or gimbalsare used to coarsely steer the narrow field of view 230 to the subject134. An adaptive optics system (not shown in FIG. 1) is then used toachieve fast, fine tracking of the subject 134 and optionally also focusadjustment for the image capture. The adaptive optics system is drivenby the eye reflection from the subject's eye 134 and/or by otherposition and distance measurement techniques. Other approaches can alsobe used. Risley prisms, liquid crystal phased arrays, real timeholograms and Bragg gratings are examples of other steering devices.Other signal sources could include glints, parallax using images or eyereflections, and time of flight lidar.

FIG. 2 is an illustration of an example iris imaging system according tothe present invention, based on retro-reflection from the eye. In thisexample, the acquisition subsystem 100 includes a light source 110, abeam splitter 115, a small “pickoff” mirror 119 and a camera 150. Theimaging subsystem 200 includes a light source 210, a beamsplitter 215, adeformable mirror 220, a beamsplitter 225, a wavefront sensor 227 and acontroller 222. It also includes a light source 248 and a camera 250.For convenience, the various light sources may be referred to as theacquisition light source 110, the WFS light source 210 and the irisimaging light source 248, respectively, to distinguish them from eachother. The iris imaging system also includes a coarse tip-tilt steeringmirror 120 controlled by controller 190, which is used as part of boththe acquisition subsystem 100 and the imaging subsystem 200. In FIG. 2,the steering mirror 120 is depicted as a line through the optical beambut, for simplicity, reflection off the steering mirror is not shown(i.e., the optical path is unfolded with respect to steering mirror120). Various lenses (or other optics) are used to collimate, focus,image or otherwise relay the optical beams throughout the system.

The acquisition subsystem 100 operates as follows. The acquisition lightsource 110 is the illumination for camera 150. Light produced by lightsource 110 reflects off beamsplitter 115, and mirror 119. Beamsplitter115 separates light produced by source 110 that is exiting the systemand light returning to the system to be imaged onto camera 150.Beamsplitter 115 could be a polarizing beamsplitter, which together witha quarterwave plate could be used to suppress back reflection andspecular reflections. Beamsplitter 115 could also be a neutralbeamsplitter (i.e., without polarization selectivity) for low cost andsimplicity. Mirror 119 combines the optical paths of the acquisitionsubsystem 100 and the imaging subsystem 200 so they are generallyaligned along a common optical axis. In this example, the two subsystemsoperate at different wavelengths, so mirror 119 is a dichroicbeamsplitter that reflects the wavelengths of the acquisition subsystem100 and passes the wavelengths of the imaging subsystem 200. Theoutgoing illumination from light source 110 then reflects off coarsesteering mirror 120 to illuminate the acquisition subsystem 100's widerfield of view 135. The field of view 135 may stare across the entirecapture volume 50 or may be scanned across the capture volume. In thisexample, the field of view 135 is not wide enough to cover the entirecapture volume in a staring mode. Rather, it is scanned across thecapture volume by steering mirror 120. Subjects within the field of view135 are represented by eyes 134, which are illuminated by theacquisition light source 110.

Eyes 134 within the field of view 135 retro-reflect light back to thecoarse steering mirror 120, which directs the light to camera 150 viamirror 119 and beamsplitter 115. Camera 150 is a wide angle camera usedto identify the general locations of eyes 134. In one implementation,the camera 150 is an electronic image sensor such as a CCD thatperiodically records discrete images of field of view 135. In oneapproach, the camera 150 records rapid sequences of images to monitorthe movement of objects 134 within the field of view 135. The signalsfrom the wide angle camera are analyzed by software (e.g., contained incontroller 190) to identify eyes, which appear as bright circular spotsdue to the retro-reflections from the eyes 134. The camera 150 operatesat the same wavelength as the illuminating source 110. Wavelengthfilters can be used to reject ambient light on the return optical path,while passing the illuminating wavelength. In addition, the light source110 can be strobed. Synchronization of the camera 150 exposures with thesource 110 strobing can also increase the isolation between imaging andguiding (or wavefront sensor) cameras. Such synchronization can alsoreduce the effects of background light contamination.

Once eyes 134 are identified, the controller 190 determines a plan forimaging the irises. Preferably, iris images of both eyes are captured(although not necessarily simultaneously), in order to increase theaccuracy of identification. In FIG. 2, the iris 134A is being imaged. Ifnecessary, the controller 190 directs the coarse steering mirror 120 tobring the eye of interest 134A within the narrower field of view for theimaging subsystem 200. As drawn in FIG. 2, the coarse steering mirror120 also steers the wide field of view 135 for the acquisition subsystem100, although this is not required. One advantage of steering theacquisition subsystem 100 and imaging subsystem 200 together is that afixed relationship between the wavefront sensor 227 and the acquisitioncamera 150 is maintained.

The imaging subsystem 200 operates as follows. WFS light source 210illuminates the eye 134A. Light produced by light source 210 reflectsoff beamsplitter 215, propagates through lens system 221 and mirror 119,and is directed by steering mirror 120 to the eye 134A. Since this lightis coming from the imaging subsystem 200, it has a narrower field ofview than the field of view 135 of the acquisition subsystem. A portionof the illuminating light enters the eye 134A, which retro-reflectslight back along the same path 120-221. The return light passes throughthe beamsplitter 215, reflects off deformable mirror 220 and is directedby beamsplitter 225 to the wavefront sensor 227. The wavefront sensor227, controller 222 and deformable mirror 220 form an adaptive opticsloop that is driven based on the retro-reflected light from the eye134A.

In one variation, polarization is used to distinguish retro-reflectedlight from a target eye 134 from glints. The illuminating light from WFSlight source 210 is polarized and beamsplitter 215 is a polarizationbeamsplitter. The beamsplitter 215 reflects the originally polarizedlight, directing it to the eye 134. A quarterwave plate placed afterbeamsplitter 215 (e.g., between beamsplitter 215 and lens 221) rotatesthe polarization by ninety degrees after a double pass (i.e., one passupon transmission from the WFS light source 210 to the eye 134A and asecond pass upon retro-reflection from the eye 134A). Glints, i.e.,reflections from smooth surfaces, generally preserve the polarization ofthe incident light and therefore will be reflected by the polarizationbeamsplitter 215 on the return path and will not pass through to thewavefront sensor 227. Such glints may include reflections from theobjective lens 221, reflections from the front of the eye 134 orglasses, and others. The retro-reflection from the retina of the targeteye 134, however, does not maintain the polarization of the incidentlight due to the structure of the eye, and therefore a portion of thislight is transmitted through the beamsplitter to the wavefront sensor227.

While adaptive optics can be used in many applications to correct forhigh order aberrations, in this case, the adaptive optics loop is usedmainly for fast tracking of the eye 134A (i.e., correction of tip/tilterrors in the wavefront) and preferably also for focus correction. Thiskeeps the iris 134A within the narrow field of view of camera 250 andalso focuses the camera (if focus correction is implemented). In thisexample, the light source 210 does not provide the primary illuminationfor camera 250. Rather, additional light sources 248 (i.e., the irisimaging light sources) provide off-axis illumination of the irises 134for camera 250. For example, LEDs in the near infrared wavelength rangecan be used. The protective pigment melanin is more transparent atlonger wavelengths. Thus, the details of the iris structure are moreeasily seen in heavily pigmented eyes by using light sources of thesewavelengths. Alternatively, any other light source could be used thatconforms to safety limits. The off-axis illumination generally resultsin higher contrast and fewer artifacts. Off-axis illumination angle alsoaffects positioning of glints which can be deleterious to theidentification accuracy. Glints can also be reduced by using polarizedillumination with polarizing filters for the iris camera 250. Inalternate approaches, illumination for camera 250 can be provided byambient lighting, visible or infrared flash, or combinations of these.

Traditional adaptive optics systems, such as those developed forastronomy, may be too large, complex and/or costly to be effectivelyused in applications such as iris imaging. However, recent advances byAOptix Technologies of Campbell, Calif., have resulted in thedevelopment of complete adaptive optics systems, including electronics,that achieve sizes smaller than a shoe box. The AOptix adaptive opticssystems require less than 25 W of power and can reliably operateunattended for extended periods of time. The small size, weight andpower and high reliability of the AOptix adaptive optics systems makethem suitable for applications such as the iris imaging applicationsdescribed herein.

In these more compact systems, the deformable mirror 220 is a deformablecurvature mirror based on applying different voltages across differentareas of a piezoelectric material, thus causing deformation. Furtherdetails for this type of deformable mirror are described and shown inU.S. Pat. No. 6,464,364, “Deformable Curvature Mirror,” filed Jan. 25,2001 and issued Oct. 15, 2002, by J. Elon Graves and Malcolm J.Northcott; U.S. Pat. No. 6,568,647, “Mounting Apparatus for DeformableMirror,” filed Jan. 25, 2001 and issued May 27, 2003, by J. Elon Gravesand Malcolm J. Northcott; and U.S. Pat. No. 6,721,510, “AtmosphericOptical Data Transmission System,” filed Jun. 16, 2001 by J. Elon Gravesand Malcolm J. Northcott. Furthermore, the wavefront sensor 227 is awavefront curvature sensor based on defocused pupil images. Furtherdetails for this type of wavefront curvature sensor are described andshown in U.S. Pat. No. 6,452,145, “Method and Apparatus for WavefrontSensing,” filed May 26, 2000 and issued Sep. 17, 2002, by J. Elon Gravesand Malcolm J. Northcott; and U.S. Pat. No. 6,721,510, “AtmosphericOptical Data Transmission System,” filed Jun. 16, 2001 by J. Elon Gravesand Malcolm J. Northcott. All of the foregoing are incorporated hereinby this reference.

In one embodiment, the iris imaging system of FIG. 2 is designed for usein airport hallways, customs checkpoints, public transportationstations, secure building lobbies, and the like. Standoff distances ofup to at least 10 meters would enable the scanning of a large room orhallway to identify the occupants. For example, a device could be placedin the vicinity of the departure and/or arrival screen in an airport.The system would then be able to identify anyone attempting to read thescreen contents.

For this specific design, the acquisition subsystem 100 has a field ofview 135 of approximately 12 degrees, resulting in a capture volume 50measuring approximately 2 m×2 m×2 m at a 10 m range (without scanning).The acquisition light source 110 is a light-emitting diode (LED) havinga wavelength in the range of 750 to 980 nm. Shorter wavelengths givebetter sensor quantum efficiency, but wavelengths longer thanapproximately 890 nm are required for invisible operation. Longerwavelengths are also possible but require more expensive (not silicon)detectors. LED sources are generally preferred. Laser sources areproblematical due to eye safety considerations, but could be used withcareful engineering. Gas discharge lamps could also be used under somecircumstances. Thermal sources such as tungsten lights and arc lampscould also be used but would be inefficient due to the requirement forwavelength filtering.

In this specific design, the illuminating wavelength used by theacquisition subsystem 100 is different than that used by the imagingsubsystem 200, so mirror 119 can be wavelength-selective to separate thelight for the acquisition subsystem 100 from that for the imagingsubsystem. The acquisition camera 150 is an infrared enhanced monochromeTV camera with a resolution of approximately 720×500 pixels. The camera150 operates at a 30 Hz frame rate.

With respect to the imaging subsystem 200, the resolution requirementsdrive the design of the iris imaging system 200. Consider a resolutionrequirement of 75 microns per pixel. Assuming diffraction limitedperformance, the required aperture diameter d is given by d=λz/r, wherez is the standoff distance and r is the required resolution. Forexample, assuming λ=0.82 μm, and z=10 m, the required aperture is 11 cm.As another example, a 100 μm resolution can be achieved at a visiblewavelength of 0.5 μm at a 10 m standoff distance with a diffractionlimited 5 cm aperture. However, infrared wavelengths are generallypreferred for iris imaging due to the enhanced contrast observed atlonger wavelengths.

The diffraction limited resolution requirement and large aperture alsolead to a limited depth of field. If the geometric image spread due tofocus depth of field is set to be less than half of the diffractionlimit, then the depth of field l is given by l=r²/λ. The 0.82 μm exampleyields a depth of field of approximately 7 mm. The 0.5 μm example yieldsa depth of field of approximately 2 cm. Depth of fields on the order ofa few millimeters or a few centimeters makes focusing on moving objectsdifficult. Hence, it is advantageous for the adaptive optics loop toimplement fast focus correction as well as fast tracking. With theadaptive optics augmented iris imaging system, images can be takenwithin a few milliseconds of identifying a target. Thus, the use ofadaptive optics can increase the speed and accuracy of image capture forapplications involving uncooperative targets.

Focus adjustment can also be achieved using other variations andapproaches. For example, a variable focus lens or deformable mirror canbe used to adjust the focus. Electro-mechanical lens positionadjustment, movement of the camera 250 and use of a variable refractiveindex element are alternate ways to adjust focus. In addition, focuswavefront sensing can be based on image contrast measurements anddithering, or by use of a dedicated focus wavefront sensor, or bymeasuring the distance to the eye using time of flight of an optical oracoustic pulse.

Continuing with the specific example described above, the WFS lightsource 210 used in the iris imaging system 200 can be chosen toilluminate the eye so that the target individual is unaware of theprocess. LEDs having wavelengths in the range of 750 to 980 nm aregenerally preferred (and greater than approximately 890 nm for invisibleoperation), but other sources can be used as described above. Fillingthe telescope aperture with the illumination light as shown in FIG. 2 isadvantageous, since it ensures that the pupil is fully illuminated bythe eye reflection. The iris imaging light sources 248 are alsopreferably LEDs. Iris imaging standards currently specify wavelengthsaround the 850 nm range.

In this example, the WFS illuminating wavelength (used by the wavefrontsensor 227) is also selected to be different from the illumination usedto image the irises by camera 250. Hence, the beamsplitter 225 isdichroic to increase efficiency. However, these separations inwavelength are not required. The different beams can be separated usingother techniques. For example, the iris imaging illumination and WFSillumination can be distinguished by time instead. The WFS LED 210 canbe flashed synchronously with a WFS chopper (not shown in FIG. 2), andthe iris imaging illumination 248 flashed to fill the dead time when thewavefront sensor 227 is not integrating signal. The iris imaging camera250 preferably is a high quality monochrome imager. Due to the highspeed tracking, this imager 250 can have a relatively small number ofpixels, for instance a standard 640×480 video imager is convenient. Forthe iris imaging camera 250, high quality, high quantum efficiency andlow signal to noise are relatively more important than resolution. Theacquisition camera 150 will generally have a separate illuminationsystem 110. If interference occurs between the acquisition illumination110, the iris imaging illumination 248 and/or the fine trackingillumination 210, various techniques can be used to provide isolation,including for example techniques based on wavelength, polarization,temporal separation and/or angular or spatial separation. Thesetechniques are described in greater detail below.

The example of FIG. 2 is based on retro-reflection from the eye. FIG. 3is an illustration of retro-reflection from the human eye. The intrinsicgeometry of the eye causes it to act as a retro-reflector. Light thatenters the eye lens 304 is focused onto the retina 314. Any lightscattered by the retina back towards the lens 404 retraces its path outof the eye. Because the retina is in the focal plane of the eye lens,light is strongly directed in the backscatter direction. As FIG. 3shows, light enters the eyeball through the pupil and reflects from theback curved surface of the retina 314. It is this back-reflection fromthe retina 314 that can be used to drive the fine tracking system in theimaging subsystem (e.g., the wavefront sensor in the adaptive opticsloop). Also, the illustration of FIG. 3 shows that the illumination neednot come from a face-on aspect to create a retro-reflection. Thus, thesubject need not stare directly into the iris imaging camera for theacquisition and imaging system to work.

FIG. 4 is an illustration of a typical reflectance spectrum of a humaneye. This graph was originally presented in the thesis of Niels Zagers,University of Utrecht. The reflectance shows a strong peak towards theinfrared. Using a wavelength of 750 nm (CD read laser wavelength), areflectivity of 4% of the white Lambertian diffuser value is expected.The back reflection property is stronger in the red and near IR (around800 nm) wavelengths, since melanin which is found in the retina, is lessabsorbing at red wavelengths. At a 750 nm or longer wavelength, thesubject would only see faint illumination since this is outside thenominal visible region. At 880 nm or longer wavelength the light sourcewill be essentially invisible.

The following example demonstrates how retro-reflected light from an eye234 can be used in closed loop operation of an adaptive optics system. Asubject at a 10 m distance can be illuminated with 0.1 mW of power tothe eye, which is well within the eye safety limit. In this example, theretro-reflected light is expected to be approximately 6.4×10⁻¹³ W/cm².Assuming a 5 cm imaging lens is used to achieve a 100 micron resolution,approximately 1.2×10⁻¹¹ W is captured on the wavefront sensor. Thiscorresponds to a photon flux of approximately 5×10⁷ photons per second.In one embodiment, a low order adaptive optics system running at arelatively slow rate is used. For example, a 19 actuator adaptive opticssystem updated at 1 KHz, provides approximately 2500 photons peractuator per update. A CCD type detector with better than 50-electronread noise and 50% quantum efficiency will provide sufficient signal tonoise ration for closed loop operation of the adaptive optics system.For comparison, better than 10-electron read noise and 90% quantumefficiency is routinely achieved for scientific grade CCD imaging. Thus,the retro-reflected light can be used to derive the feedback signal tosupport adaptive optics-assisted fine tracking and imaging.

Advantages of using the eye as a retro-reflector to drive the wavefrontsensor include low cost and long range. The low cost is due to theability to use an inexpensive silicon detector as the wavefront sensorand inexpensive LEDs as light sources. An adequate signal is achievedeven at long ranges due to the strong directionality of theretro-reflection. However, the retinal retro-reflection does not appearas a point source, so higher dynamic range detectors are used togenerate an accurate wavefront signal.

In the example of FIG. 2, the reflection from the eye was a retinalretro-reflection. Alternatively, the front surface of the eye acts as apartial mirror with about 4% reflectivity. Reflections from this surfaceform a glint that can be used to steer the imaging subsystem 200 insteadof the retro-reflection. For example, the system of FIG. 2 can bemodified so that the light source 210 illuminates eye 134A, but thewavefront sensor 227 is driven by a glint reflection from the eye ratherthan a retro-reflection. Since glints can be produced by off-axisillumination, the light source 210 can be moved off-axis or even outsidethe telescope 221 for the imaging subsystem 200. In the example of FIG.5, the light source 210 is replaced by an external light source 212.This source 212 is positioned at locations more like illuminators 248but still produces a glint for telescope 221. In addition, the glintlooks like a de-magnified image of the light source, so it tends to bemore like a point source. A resulting advantage is that the size andshape of the glint is not a strong function of the distance to thesubject.

One advantage of driving the wavefront sensor from the glint of theeyeball is that there is no limitation on distance over which glintsfrom eyeballs can be used. Also, a point-like source does not require awavefront sensor with a high dynamic range. However, glints return lesslight than retro-reflections from eyes, so more wavefront sensorsensitivity or a higher illumination flux may be required.

As describe above, illumination can be used for three principle purposesin the iris imaging system. To reiterate, light can be used by theacquisition subsystem to identify the approximate location of subjectswithin a capture volume. Light can be used by the fine trackingsubsystem to drive the adaptive optics loop to steer the camera to theeye. Light can also be used to illuminate the iris for imaging by theimaging subsystem. In the example of FIG. 2, the imaging system hasthree separate light sources for these purposes: acquisition lightsource 110, WFS light source 210 and the iris imaging light source 248.Note that each “light source” could include multiple devices, such asbanks of LEDs arranged at various locations. The term “light source” isnot intended to be limited to a single device.

In some variations, less than three separate light sources are usedeither because one light source is not needed at all or because onelight source functions for more than one purpose. For example,acquisition subsystem 100 may not be based on cameras at all, but ratheron position sensors of various kinds, thus obviating acquisition lightsource 110. As another example, imaging subsystem 200 may not be basedon an eye reflection driving an adaptive optics loop, but rather onother position and distance measurement techniques that obviate WFSlight source 210.

In other examples, the light used for one of these three purposes, i.e.,acquisition, driving the adaptive optics loop, imaging, is also used foranother of these purposes. For example, one light source can be used forboth acquisition and driving the adaptive optics loop.

In another example, one light source can be used for both acquisitionand imaging. In yet another example, one light source can be used forboth driving the adaptive optics loop and imaging. In somecircumstances, one light source can be used for acquisition, driving theadaptive optics loop, and imaging.

In some implementations, the light that is provided for one purpose,i.e., acquisition, driving the adaptive optics loop, or imaging, mayinterfere with another purpose. In the example configuration shown inFIG. 2, acquisition light 110 results in on-axis illumination of the eyebecause mirror 119 combines the optical paths of the acquisitionsubsystem 100 and the imaging subsystem 200. The on-axis illumination ofthe eye by acquisition light source 110 results in a retinalretro-reflection that also returns on-axis. This is desirable for theacquisition subsystem 100 because this retro-reflection is used bycamera 150 to identify the approximate location of eyes. However, theretinal retro-reflection is many times more efficient than the scatteredlight return from the iris tissue used to drive the camera 250. Thus, ifthe retro-reflection is not filtered from these other subsystems, theretinal return can be so strong that it causes unwanted blooming on thecamera 250 and decreases image contrast with scattered light in theoptics. Thus, in some further variations, light for one purpose, i.e.,acquisition, driving the adaptive optics loop, or imaging, isdifferentiated from other light for another purpose by wavelength, angle(i.e., propagation direction), spatial location, time, polarization,electrical modulation, or some combination of these attributes. Byseparating various light sources, the effects of blooming, noise,decreased image contrast, and background light contamination can bereduced. However, on axis illumination can be used provided sufficientcare is taken with the optical design to minimize ghost reflections, anda sensor is used which is not prone to bleeding. In this case the strongretroreflection (red eye) present in the iris image may be removed bysoftware processing. An advantage of this arrangement is that thepresence of a strong red-eye return allows very accurate determinationof the pupil boundary. Fine structure identified at the edge of thepupil boundary may be used as a supplementary biometric. A significantadvantage of on-axis, or near on-axis illumination is that it can besteered by the same steering element that is used to steer the irisimager. Using a common steering element allows for targeted small areaillumination with a high local illumination flux, using a minimum numberof steering elements, without the need for parallax correction thatwould be required with a separate illumination steering device. Inimplementations with a separate illumination steering device, softwarecorrection of parallax errors can be used.

Continuing the example of FIG. 2, the returning retro-reflection used bythe acquisition subsystem 100 can be filtered from the iris imagingsubsystem 200 in a number of different ways. In the specific example ofFIG. 2, the acquisition light source 110 is a different wavelength anddichroic mirror 119 filters the retro-reflection based on wavelength.Differentiation by polarization can be implemented in a similar manner.Strobing can be used to filter the retro-reflection on the basis oftime. For example, the acquisition light source 110 can be strobed onwhenever the iris imaging camera 250 is strobed off, for example by useof a chopper. FIG. 6, discussed in more detail below, shows an exampleof spatial differentiation, where the acquisition subsystem 100 and irisimaging subsystem 200 use different apertures. In angulardifferentiation, the two subsystems would share a common aperture butthe relevant light would be propagating in different direction. In“modulation” differentiation, the acquisition light source 110 mightencode a certain modulation on the outgoing light beam. Assuming thatthe return beam continued to carry this modulation, it could beelectrically filtered out of the return signal, or removed usingsoftware processing. A generalization of this process can be used toreduce the contamination of the iris image by reflections from thecorneal surface. The corneal reflections typically come from reflectionof other light sources, windows and structures in the hemisphere infront of the cornea. If a sequence of iris images is taken withdifferent iris illumination levels, then the corneal reflections may beisolated by suitable algebraic processing of the resulting images. In asimple illustration of the method, two images may be taken, one with theiris illumination turned on, and one with the illumination turned off.In this case, subtracting the image with no iris illumination from theimage where illumination was turned on would remove features due toreflections from the cornea, since these reflections would be the samein both images.

Generally, if interference occurs between the acquisition illumination110, the fine tracking illumination 210, and/or the iris imagingillumination 248, various techniques can be used to provide isolation,including for example techniques based on spatial or angular separation,wavelength, temporal separation, polarization, and/or modulation. Insome embodiments, two or more of these techniques can be applied incombination within one imaging system. Also in some embodiments, two ormore of these techniques can be applied to differentiate two sources ofillumination within one imaging system. The following section describesand provides examples of each of these techniques. Similar techniquescan be used to reduce the effects of external illumination on thequality of the iris images. Image contamination may come from buildinglighting, and other sources of light not under direct control of theiris imaging device.

Spatial separation can be used to differentiate illumination for onepurpose so as to reduce the negative effects caused by interferencebetween light sources. FIG. 6 is an example of an imaging system thatuses retinal retroreflection from acquisition light source 110 foracquisition purposes, and uses glint reflection of WFS light 212 fromthe corneal surface for driving the adaptive optic loop, and usesscattering of the imaging light 248 from the iris structure for imagingthe iris. Light source 212 may optionally be present, for example ifglint from light source 248 is used to drive the adaptive optics loop.This configuration uses physical separation of the apertures for theacquisition subsystem and the imaging subsystem to reduce interference.As shown, the aperture for the acquisition subsystem is physicallyseparated from the aperture for the iris imaging subsystem. Therefore,the retinal retroreflection from acquisition source 110 will retrace itspath out of the eye 134A back to the aperture for the acquisitionsubsystem and to acquisition camera 150. More importantly, this pathdoes not enter the aperture for the iris imaging system, thus reducinginterference by acquisition light 110 in imaging.

In addition, note that in this example, light sources 212 and 248 areoff-axis, which generally helps bring out topographic relief and resultsin higher contrast of iris features. In addition, unlike on-axisillumination, off-axis illumination results in glints from the cornealsurface falling outside the center of the retinal return. The contrastof the glint against the background can be dramatically reduced if itfalls within the retinal return, thus degrading the overallsignal-to-noise ratio for tracking and focus of the adaptive opticsloop. However for the purposes of iris imaging, it is better to have theglint fall within the pupil where it will not occlude any of the irisstructure. In the latter case the wavefront sensing algorithm will needto deal with any red-eye background that surrounds the glint. FIG. 6shows an example of isolation by spatial separation, but other examplesof spatial and/or angular separation are also possible to reduce oreliminate interference between illumination sources for differentpurposes.

Note that the surface of the cornea is roughly spherical and thus formsa virtual image of the illumination source as seen by the camera (theglint image). The geometry of the cornea is such that the position ofthe glint virtual image is very close to the plane of the iris. Thus, ifthe system focuses on the glint, it will be focusing to the correctplane to optimize iris imaging. Likewise if the wavefront information isderived from the pupil retro-reflection, the wavefront information willbe associated with the edge of the pupil, which is also in the plane ofthe iris.

Wavelength can also be used to differentiate illumination and reduceinterference between light sources. As described above with reference toFIG. 2, the WFS light source 210 can be selected to be different fromthe iris imaging illumination 248. Hence, the beamsplitter 225 isdichroic to increase efficiency. If stealth is important, wavelengthranges can still be chosen from the non-visible portion of the spectrum.LEDs having wavelengths in the range of 750 to 980 nm are generallypreferred (and greater than approximately 890 nm for invisibleoperation), but other sources can be used as described above. Theprotective pigment melanin is more transparent at longer wavelengths.Thus, the details of the iris structure are more easily seen in heavilypigmented eyes by using light sources of these wavelengths. FIGS. 7 and8 show additional examples of differentiation by wavelength. In theseexamples, acquisition light source 110 can be selected to have adifferent wavelength from light source 210, 212 and/or imaging lightsource 248. Thus, element 719 in FIGS. 7 and 8 can be dichroic so as toselectively reflect the wavelength of acquisition light source 110 toprevent or reduce interference with WFS tracking or iris imaging.

Temporal separation can also be used to reduce interference, backgroundcontamination, and corneal reflections caused by other light sources.For example, as discussed above with reference to FIG. 2, acquisitionlight source 110 can be strobed and synchronized with the exposures ofcamera 150. As another example, the iris imaging illumination and WFSillumination can also be distinguished by time. As discussed above, theWFS LED 210 can be flashed synchronously with a WFS chopper (not shownin FIG. 2), and the iris imaging illumination 248 flashed to fill thedead time when the wavefront sensor 227 is not integrating signal. Thus,the iris imaging will occur when the WFS light source 210 is notemitting light, and the adaptive optics loop will be driven when theiris imaging illumination 248 is not emitting light. FIG. 9 shows twoexamples of strobing patterns that can be used to temporally separatelight sources. In Example A, acquisition light source 110 and camera150, WFS light source 210 and WFS 227; and iris imaging light source 248and camera 250 are on in a simple, sequential pattern. In Example B,acquisition light source 110 is only on once for every two times WFSlight source 210 and iris imaging light source 248 are on. Example B maybe used, for example, when two iris images are desired for every eyewithin capture volume 50. Many other strobing patterns are alsopossible, including patterns where time periods are not all of equallength, and patterns where more than one light source is on during oneor more time periods.

Polarization can also be used to differentiate light for one purposefrom other light. As discussed above, in one variation, polarization isused to distinguish retro-reflected light from a target eye 134 fromglints. For example, beamsplitter 115 in FIG. 2 can be a polarizingbeamsplitter, which together with a quarterwave plate could be used tosuppress back reflection and specular reflections for acquisition camera150. Glints, which generally preserve polarization, can also bedifferentiated (either reduced or enhanced relative to other light) byusing polarized illumination and polarization optics.

Modulation schema can also be applied to light sources so that therespective contributions from each light source can be differentiatedfrom a combination of multiple light sources detected by a WFS orcaptured by a camera. Referring now to FIG. 2, for example, nomodulation is applied to iris imaging light source 248, but a modulationsignal may be applied to WFS light 210 so that the retroreflected lightfrom eye 134 will maintain the same modulation. In this case, WFS 227can contain additional electronic filters to separate the modulatedsignal stemming from WFS light 210 and the unmodulated signal stemmingfrom iris imaging light source 248, thus separating the contributionfrom the WFS light source 210 retroreflected from eye 134 and thecontribution from iris imaging illumination 248. Alternatively, distinctmodulation schemes can be applied to each light source in an irisimaging system

Although the detailed description contains many specifics, these shouldnot be construed as limiting the scope of the invention but merely asillustrating different examples and aspects of the invention. It shouldbe appreciated that the scope of the invention includes otherembodiments not discussed in detail above. Various other modifications,changes and variations which will be apparent to those skilled in theart may be made in the arrangement, operation and details of the methodand apparatus of the present invention disclosed herein withoutdeparting from the spirit and scope of the invention as defined in theappended claims. For example, the fine tracking system within theimaging subsystem may use a deformable mirror to steer the camera fromeye to eye, but the deformable mirror may be driven by feedback otherthan from a wavefront sensor. For example LIDAR, radar and other rangefinding technologies, image parallax or image contrast measurements andpattern recognition can be used to drive the deformable mirror.Therefore, the scope of the invention should be determined by theappended claims and their legal equivalents.

1. An iris imaging system comprising: an acquisition subsystem comprising: a first light sources for producing a first optical beam, and first optics having a first aperture, for directing the first optical beam through the first aperture to illuminate a moving object within a capture volume and further for receiving through the first aperture a retro-reflection of the first optical beam from the moving object to identify an approximate location of the object; and an imaging subsystem comprising: a second light source for producing a second optical beam to illuminate the moving object, second optics having a second aperture spatially separated from the first aperture, so that the second aperture does not receive the retro-reflection of the first optical beam, the second optics for producing an image of the illuminated moving object, and a camera for capturing the image of the moving object with sufficient resolution for image recognition of the image.
 2. The iris imaging system of claim 1 wherein the first light source comprises a plurality of light sources.
 3. The iris imaging system of claim 1 wherein the second light source comprises a plurality of light sources.
 4. The iris imaging system of claim 1 wherein the first light source comprises a bank of LED devices.
 5. The iris imaging system of claim 1 wherein the second light source comprises a bank of LED devices. 